The present invention is directed to integrated circuits. More particularly, the invention provides systems and methods for voltage regulation and current regulation. Merely by way of example, the invention has been applied to a power conversion system. But it would be recognized that the invention has a much broader range of applicability.
FIG. 1 is a simplified diagram showing a conventional flyback power conversion system. The power conversion system 100 includes a primary winding 110, a secondary winding 112, a power switch 120, a current sensing resistor 130, a rectifying diode 160, a capacitor 162, an isolated feedback component 114, and a controller 170. The controller 170 includes an under-voltage-lockout component 172, a pulse-width-modulation generator 174, a gate driver 176, a leading-edge-blanking (LEB) component 178, and an over-current-protection (OCP) component 180. For example, the power switch 120 is a bipolar transistor. In another example, the power switch 120 is a field effect transistor.
As shown in FIG. 1, the power conversion system 100 uses a transformer including the primary winding 110 and the secondary winding 112 to isolate an AC input voltage 102 on the primary side and an output voltage 104 on the secondary side. Information related to the output voltage 104 is processed by the isolated feedback component 114 which generates a feedback signal 154. The controller 170 receives the feedback signal 154, and generates a gate-drive signal 156 to turn on and off the switch 120 in order to regulate the output voltage 104.
To achieve good output current control, the power conversion system 100 often needs additional circuitry in the secondary side, which usually results in high cost. Moreover, the required output current sensing resistor in the secondary side usually reduces the efficiency of the power conversion system 100.
FIG. 2 is a simplified diagram showing another conventional flyback power conversion system. The power conversion system 200 includes a primary winding 210, a secondary winding 212, an auxiliary winding 214, a power switch 220, a current sensing resistor 230, two rectifying diodes 260 and 268, two capacitors 262 and 270, and two resistors 264 and 266. For example, the power switch 220 is a bipolar transistor. In another example, the power switch 220 is a MOS transistor.
Information related to the output voltage 250 can be extracted through the auxiliary winding 214 in order to regulate the output voltage 250. When the power switch 220 is closed (e.g., on), the energy is stored in the secondary winding 212. Then, when the power switch 220 is open (e.g., off), the stored energy is released to the output terminal, and the voltage of the auxiliary winding 214 maps the output voltage on the secondary side as shown below.
                              V          FB                =                                            R              2                                                      R                1                            +                              R                2                                              ×                      V            aux                                              (                  Equation          ⁢                                          ⁢          1                )            where VFB represents a feedback voltage 274, and Vaux represents a voltage 254 of the auxiliary winding 214. R1 and R2 represent the resistance values of the resistors 264 and 266 respectively.
A switching period of the switch 220 includes an on-time period during which the switch 220 is closed (e.g., on) and an off-time period during which the switch 220 is open (e.g., off). For example, in a continuous conduction mode (CCM), a next switching cycle starts before the completion of a demagnetization process associated with the transformer including the primary winding 210 and the secondary winding 212. Thus, the actual length of the demagnetization process before the next switching cycle starts is limited to the off-time period of the switch. In another example, in a discontinuous conduction mode (DCM), a next switching cycle does not start until a period of time after the demagnetization process has completed. In yet another example, in a quasi-resonant (QR) mode or a critical conduction mode (CRM), a next switching cycle starts shortly after the completion of the demagnetization process.
FIG. 3(A) is a simplified conventional timing diagram for the flyback power conversion system 200 that operates in the continuous conduction mode (CCM). The waveform 302 represents the voltage 254 of the auxiliary winding 214 as a function of time, the waveform 304 represents a secondary current 278 that flows through the secondary winding 212 as a function of time, and the waveform 306 represents a primary current 276 that flows through the primary winding 210 as a function of time.
For example, a switching period, Ts, starts at time t0 and ends at time t2, an on-time period, Ton, starts at the time t0 and ends at time t1, and an off-time period, Toff, starts at the time t1 and ends at the time t2. In another example, t0≦t1≦t2.
During the on-time period Ton, the power switch 220 is closed (e.g., on), and the primary current 276 flows through the primary winding 210 and increases from a magnitude 308 (e.g., Ipri—0 at t0) to a magnitude 310 (e.g., Ipri—p at t1) as shown by the waveform 306. The energy is stored in the secondary winding 212, and the secondary current 278 is at a low magnitude 312 (e.g., approximately zero) as shown by the waveform 304. The voltage 254 of the auxiliary winding 214 keeps at a magnitude 314 (e.g., as shown by the waveform 302).
At the beginning of the off-time period Toff (e.g., at t1), the switch 220 is open (e.g., off), the primary current 276 is reduced from the magnitude 310 (e.g., Ipri—p) to a magnitude 316 (e.g., approximately zero) as shown by the waveform 306. The energy stored in the secondary winding 212 is released to the output load. The secondary current 278 increases from the magnitude 312 (e.g., approximately zero) to a magnitude 318 (e.g., Isec—p) as shown by the waveform 304. The voltage 254 of the auxiliary winding 214 increases from the magnitude 314 to a magnitude 320 (e.g., as shown by the waveform 302).
During the off-time period Toff, the switch 220 remains open, the primary current 276 keeps at the magnitude 316 (e.g., approximately zero) as shown by the waveform 306. The secondary current 278 decreases from the magnitude 318 (e.g., Isec—p) to a magnitude 322 (e.g., Isec—2 at t2) as shown by the waveform 304. The voltage 254 of the auxiliary winding 214 decreases from the magnitude 320 to a magnitude 324 (e.g., as shown by the waveform 302).
At the end of the off-time period Toff (e.g., t2), a next switching cycle starts before the demagnetization process is completed. The residual energy reflects back to the primary winding 210 and appears as an initial primary current, Ipri—0, at the beginning of the next switching cycle.
For example, the primary current 276 and the secondary current 278 satisfy the following equations:Isec—p=N×Ipri—p  (Equation 2)Isec—2=N×Ipri—0  (Equation 3)where Isec—p represents the secondary current 278 when the off-time period Toff starts, and Isec—2 represents the secondary current 278 when the off-time period Toff ends. Additionally, Ipri—p represents the primary current 276 when the on-time period Ton ends, Ipri—0 represents the primary current 276 when the on-time period Ton starts, and N represents a turns ratio between the primary winding 210 and the secondary winding 212.
The output current 252 can be determined based on the following equation:
                              I          out                =                              1            2                    ×                      1            T                    ×                                    ∫              0              T                        ⁢                                          (                                                      I                    sec_p                                    +                                      I                                          sec_                      ⁢                      2                                                                      )                            ×                                                T                  demag                                                  T                  s                                            ⁢                                                          ⁢                              ⅆ                t                                                                        (                  Equation          ⁢                                          ⁢          4                )            where Iout represents the output current 252, T represents an integration period, Ts represents a switching period, and Tdemag represents the duration of the demagnetization process within the switching period. For example, Tdemag is equal to the off-time period Toff in the CCM mode.
Combining the equations 2, 3 and 4, one can obtain the following equation.
                              I          out                =                              N            2                    ×                      1            T                    ×                                    ∫              0              T                        ⁢                                          (                                                      I                    pri_p                                    +                                      I                                          pri_                      ⁢                      0                                                                      )                            ×                                                T                  demag                                                  T                  s                                            ⁢                                                          ⁢                              ⅆ                t                                                                        (                  Equation          ⁢                                          ⁢          5                )            
Referring to FIG. 2, the resistor 230, in combination with other components, generates a current-sensing voltage signal 272 (e.g., Vcs) which is related to the primary current 276. For example, the output current 252 can be determined according to the following equation:
                              I          out                =                              N            2                    ×                      1                                          R                s                            ×              T                                ×                                    ∫              0              T                        ⁢                                          (                                                      V                                          cs                      ⁢                                                                                          ⁢                      1                                                        +                                      V                                          cs                      ⁢                                                                                          ⁢                      0                                                                      )                            ×                                                T                  demag                                                  T                  s                                            ⁢                                                          ⁢                              ⅆ                t                                                                        (                  Equation          ⁢                                          ⁢          6                )            where Vcs0 represents the current-sensing voltage signal 272 when an on-time period starts during a switching cycle, Vcs1 represents the current-sensing voltage signal 272 when the on-time period ends during the switching cycle, and Rs represents the resistance of the resistor 230.
In another example, the output current 252 can be determined based on the following equation:
                                          I            out                    =                                    N              2                        ×                          1                                                R                  s                                ×                K                                      ×                                                            ∑                  K                                1                            ⁢                                                (                                                                                    V                                                  cs                          ⁢                                                                                                          ⁢                          1                                                                    ⁡                                              (                        n                        )                                                              +                                                                  V                                                  cs                          ⁢                                                                                                          ⁢                          0                                                                    ⁡                                              (                        n                        )                                                                              )                                ×                                                                            T                      demag                                        ⁡                                          (                      n                      )                                                                                                  T                      s                                        ⁡                                          (                      n                      )                                                                                                          ⁢                                                      (                  Equation          ⁢                                          ⁢          7                )            where n corresponds to the nth switching cycle, Vcs0(n) represents a magnitude of the current-sensing voltage signal 272 when an on-time period Ton starts in the nth switching cycle, and Vcs1(n) represents a magnitude of the current-sensing voltage signal 272 when the on-time period ends in the nth switching cycle. Additionally, K represents the number of switching cycles that are included in the calculation. For example, K can be infinite; that is, the calculation of Equation 7 can include as many switching cycles as needed. As shown in Equations 6 and 7, the output current 252 may be regulated (e.g., be kept constant) based on information associated with the current-sensing voltage signal 272.
FIG. 3(B) is a simplified conventional timing diagram for the flyback power conversion system 200 that operates in the discontinuous conduction mode (DCM). The waveform 332 represents the voltage 254 of the auxiliary winding 214 as a function of time, the waveform 334 represents a secondary current 278 that flows through the secondary winding 212 as a function of time, and the waveform 336 represents a primary current 276 that flows through the primary winding 210 as a function of time.
For example, as shown in FIG. 3(B), a switching period, Ts starts at time t3 and ends at time t6, an on-time period, Ton, starts at the time t3 and ends at time t4, a demagnetization period, Tdemag starts at the time t4 and ends at time t5, and an off-time period, Toff, starts at the time t4 and ends at the time t6. In another example, t3≦t4≦t5≦t6. In DCM, the off-time period, Toff, is much longer than the demagnetization period, Tdemag.
During the demagnetization period Tdemag, the switch 220 remains open, the primary current 276 keeps at a magnitude 338 (e.g., approximately zero) as shown by the waveform 336. The secondary current 278 decreases from a magnitude 340 (e.g., Isec—p at t4) as shown by the waveform 334. The demagnetization process ends at the time t5 when the secondary current 278 has a low magnitude 342 (e.g., zero). The secondary current 278 keeps at the magnitude 342 for the rest of the switching cycle.
A next switching cycle starts after the completion of the demagnetization process (e.g., at the time t6). For example, little residual energy reflects back to the primary winding 210 and the primary current 276 (e.g., Ipri—0 at t6) at the beginning of the next switching cycle has a low magnitude 344 (e.g., zero).
FIG. 3(C) is a simplified conventional timing diagram for the flyback power conversion system 200 that operates in the quasi-resonant (QR) mode or the critical conduction mode (CRM). The waveform 352 represents the voltage 254 of the auxiliary winding 214 as a function of time, the waveform 354 represents a secondary current 278 that flows through the secondary winding 212 as a function of time, and the waveform 356 represents a primary current 276 that flows through the primary winding 210 as a function of time.
For example, as shown in FIG. 3(C), a switching period, Ts, starts at time t7 and ends at time t10, an on-time period, Ton, starts at the time t7 and ends at time t8, a demagnetization period, Tdemag starts at the time t8 and ends at time t9, and an off-time period, Toff, starts at the time t8 and ends at the time t10. In another example, t7≦t8≦t9≦t10. In the CRM mode, the demagnetization period, Tdemag, is slightly shorter than the off-time of the switch, Toff.
The demagnetization process ends at the time t9 when the secondary current 278 has a low magnitude 358 (e.g., zero). The secondary current 278 keeps at the magnitude 358 for the rest of the switching cycle. A next switching cycle starts (e.g., at t10) shortly after the completion of the demagnetization process. The primary current 276 has a low magnitude 360 (e.g., zero) at the beginning of the next switching cycle.
The power conversion system 200 often cannot achieve satisfactory dynamic responses with low standby power when the output load changes from no load to full load. Hence, it is highly desirable to improve techniques for voltage regulation and current regulation of a power conversion system.